The present invention relates to electrical isolators and in particular to a microelectromechanical system (MEMS) device providing electrical isolation in the transmission of electrical signals while limiting motion-induced noise.
Electrical isolators are used to provide electrical isolation between circuit elements for the purposes of voltage level shifting, electrical noise reduction, and high voltage and current protection.
Circuit elements may be considered electrically isolated if there is no path in which a direct current (DC) can flow between them. Isolation of this kind can be obtained by capacitive or inductive coupling. In capacitive coupling, an electrical input signal is applied to one plate of a capacitor to transmit an electrostatic signal across an insulating dielectric to a second plate at which an output signal is developed. In inductive coupling, an electrical input signal is applied to a first coil to transmit an electromagnetic field across an insulating gap to a second coil, which generates the isolated output signal. Both such isolators essentially block steady state or DC electrical signals.
Such isolators, although simple, block the communication of signals that have significant low frequency components. Further, these isolators can introduce significant frequency dependent attenuation and phase distortion in the transmitted signal. These features make such isolators unsuitable for many types of signals including many types of high-speed digital communications.
In addition, it is sometimes desirable to provide high voltage ( greater than 2 kV) isolation between two different portions of a system, while maintaining a communication path between these two portions. This is often true in industrial control applications where it is desirable to isolate the sensor/actuator portions from the control portions of the overall system. It is also applicable to medical instrumentation systems, where it is desirable to isolate the patient from the voltages and currents within the instrumentation.
The isolation of digital signals is frequently provided by optical isolators. In an optical isolator, an input signal drives a light source, typically a light emitting diode (LED) positioned to transmit its light to a photodiode or phototransistor through an insulating but transparent separator. Such a system will readily transmit a binary signal of arbitrary frequency without the distortion and attenuation introduced by capacitors and inductors. The optical isolator further provides an inherent signal limiting in the output through saturation of the light receiver, and signal thresholding in the input, by virtue of the intrinsic LED forward bias voltage.
Nevertheless, optical isolators have some disadvantages. They require a relatively expensive gallium arsenide (GaAs) substrate that is incompatible with other types of integrated circuitry and thus optical isolators often require separate packaging and assembly from the circuits they are protecting. The characteristics of the LED and photodetector can be difficult to control during fabrication, increasing the costs if unit-to-unit variation cannot be tolerated. The power requirements of the LED may require signal conditioning of the input signal before an optical isolator can be used, imposing yet an additional cost. While the forward bias voltage of the LED provides an inherent noise thresholding, the threshold generally cannot be adjusted but is fixed by chemical properties of the LED materials. Accordingly, if different thresholds are required, additional signal conditioning may be needed. Finally, the LED is a diode and thus limits the input signal to a single polarity unless multiple LEDs are used.
It is common to process analog electrical signals using digital circuitry such as microprocessors. In such situations, the analog signal may be periodically sampled and the samples converted into digital words input by an analog-to-digital converter (A/D) to and processed by the digital circuitry. Conversely, digital words produced by the digital circuitry may be converted into an analog signal through the use of a digital-to-analog converter (D/A) to provide a series of analog electrical values that may be filtered into a continuous analog signal. Isolation of such signals at the interface to the digital circuitry is often desired and may be performed by placing an optical isolator in series with the electrical signal representing each bit of the relevant digital word after the A/D converter and before the D/A converter. Particularly in the area of industrial controls where many isolated analog signals must be processed and output, a large number of optical isolators are required rendering the isolation very costly or impractical.
The present invention provides a mechanical isolator manufactured using MEMS techniques and suitable for transmitting analog or digital signals. The isolator uses a specially fabricated microscopic beam supported on a substrate and whose ends are insulated from each other. One end of the beam is connected to a microscopic actuator, which receives a user input signal to move the beam based on that signal. The other end of the beam is attached to a sensor detecting movement of the beam to provide a corresponding value.
Acceleration of the substrate, which might move the beam in the absence of a user signal, is compensated for by fabricating a second identical beam that measures inertial force and removes it from the signal. This technique can be used generally not just with isolators but also with any MEMS device in which forces or movement caused by acceleration of the substrate must be canceled. In addition, this approach also applies to other common mode noise sources other than acceleration or inertia; such as: temperature, pressure, etc.
Specifically then, the present invention provides a microelectromechanical system (MEMS) with reduced inertial sensitivity. The invention includes a substrate and a first element supported from the substrate for movement relative to the substrate with respect to an axis. A first actuator is attached to the first element to exert a force thereupon dependent upon a parameter to be measured and urging the element toward a second position. The device also includes a second element supported from the substrate also for movement with respect to the axis. A sensor assembly communicates with the first and second elements to detect movement of the first and second elements and to provide an output subtracting their movements so as to be less sensitive to substrate acceleration or other common mode noise.
Thus it is one object of the invention to provide a MEMS sensor with reduced sensitivity to acceleration interfering with measurement of the desired parameter. The small size of the MEMS device allows two matched elements to be fabricated in close proximity to each other so as to be identical and to experience the same inertial forces so that one may provide an inertial reference signal that can be used to cancel the inertial contribution to the parameter measurement.
The second element may not have an input signal applied or an actuator or functioning actuator so as to detect only inertial forces or it may include a functional actuator which exerts a force upon the second element dependent upon the parameter to be measured but urging the second element in the opposite direction as the first element.
Thus it is another object of the invention to permit a simple cancellation, which reduces inertial noise, or a more sophisticated cancellation that reduces inertial noise while also boosting the desired signal.
The parameter may be an electrical signal and the second and first actuators may receive the input electrical signal related to the parameter and exert a force dependent on the input electrical signal. In this case, the device may include an inverting circuit receiving the parameter electrical signal and producing an inverted electrical signal for the second actuator.
Thus it is another object of the invention to permit the inertial noise cancellation with identical MEMS structures simply by inverting an electrical signal to one MEMS structure so that it operates in the opposite direction.
The MEMS device may include a second actuator attached to the second element but not communicating with the parameter to be measured and thus not exerting a force thereupon dependent upon the parameter to be measured.
Thus it is another object of the invention to provide for virtually identical MEMS structures, including actuators, so as to be equally sensitive to inertial noise.
The foregoing objects and advantages may not apply to all embodiments of the inventions and are not intended to define the scope of the invention for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment also does not define the scope of the invention and reference must be made therefore to the claims for this purpose.